ECC is useless and more expensive. ECC is useful only for servers so that when it crashes it can recover some data.
So why all workstations use this type of memory. Servers use ECC registered, workstations use ECC unbuffered.
I'm sorry, I don't know the difference between registered and unbuffered ECC, but I can tell you that almost every high-end workstation has ECC disabled in the BIOS. Which makes having ECC memory kind of useless. However, I would defintely get the best memory recommended for your motherboard and processor. If it's ECC, then so beit. If it's not, fine. Just don't d_ck around with cheap memory, becaue your machine could last for a very long time. I'm typing this on a Dell PII 333mhz with 386 MB of Kingston ECC memory (because that's what was recommended for the Intel's OEM Dell 440LX motherboard). BTW, ECC is on it's default setting (off). I didn't get cheap equipment and everything is working fine 9 years later!
You will notice that with Intel's new 875-P Chipset coming out that is offering 800 Mghz Front Bus Speeds, they have dropped RAMBUS Memory altogether. As a result, there is a FIRE SALE going on with PC1066 RDRAM 533 Front Side Bus at 2.4 and up on Dell Dimension 8250's. The other day, I saw one for $750 on the Refurbished Outlet and that included FREE SHIPPING, with 512 Mb of memory! Unreal.